[Illustration:  Fig. 171.  Principle of Selection by Polarity]

Four-Party Line. From the two foregoing two-party line systems it is evident that a four-party line system may be readily obtained, that is, by employing two oppositely polarized biased bells on each side of the metallic circuit.  The selection of any of the four bells may be obtained, choosing between the pairs connected, respectively, with the two limbs of the line, by choosing the limb on which the current is to be sent, and choosing between the two bells of the pair on that side of the line by choosing which polarity of current to send.

Such a four-party line system is shown in Fig. 172.  In this the generators are not shown, but the wires leading from the four keys are shown marked plus or minus, according to the terminal of the generator to which they are supposed to be connected.  Likewise the two bells connected with the lower side of the line are marked positive and negative, as are the two bells connected with the upper side of the line.  From the foregoing description of Figs. 170 and 171, it is clear that if key K^{1} is pressed the bell at Station A will be rung, and that bell only, since the bells at Station C and Station D are not in the circuit and the positive current sent over the lower side of the line is not of the proper polarity to ring the bell at Station B.

The system shown in Fig. 172 is subject to one rather grave defect.  In subsequent chapters it will be pointed out that in common-battery systems the display of the line signal at the central office is affected by any one of the subscribers merely taking his receiver off its hook and thus establishing a connection between the two limbs of the metallic circuit.  Such common-battery systems should have the two limbs of the line, normally, entirely insulated from each other.  It is seen that this is not the case in the system just described, since there is a conducting path from one limb of the line through the two bells on that side to ground, and thence through the other pair of bells to the other limb of the line.  This means that unless the resistance of the bell windings is made very high, the path of the signaling circuit will be of sufficiently low resistance to actuate the line signal at the central office.

[Illustration:  Fig. 172.  Four-Party Polarity Selection]

It is not feasible to overcome this objection by the use of condensers in series with the bells, as was done in the system shown in Fig. 170, since the bells are necessarily biased and such bells, as may readily be seen, will not work properly through condensers, since the placing of a condenser in their circuit means that the current which passes through the bell is alternating rather than pulsating, although the original source may have been of pulsating nature only.

[Illustration:  Fig 173.  Standard Polarity System]